1. Field of the Invention
The present invention relates to control actuators for aircraft, particularly helicopters and, more particularly, to improvements in a strapdown servoactuator.
2. Description of the Prior Art
The installation of an automatic Stability Augmentation System (SAS) or an Automatic Flight Control System (AFCS) as original or retrofit aircraft equipment, particularly in helicopters, normally comprises a number of separated components usually located at separate and convenient points in the control rigging. Such components include a series actuator, usually contained within a control rod for increasing or decreasing its length in accordance with an electrical command from the SAS or AFCS computer, automatic and manual control authority limit stops, stick position transducers, an artificial feel spring mechanism having detent switches and a manual and/or automatic trim actuator and brake mechanism, and the like. In the retrofit of such actuators and associated components particularly in helicopters, the installation is more difficult, involving costly and time consuming modifications to the existing manual flight controls and linkage systems. As the above items are typically required for each axis of control, the SAS and/or AFCS original installation and retrofit becomes extremely difficult and time consuming adding not only to the aircraft's over-all-weight but also, significantly, to its total cost of ownership, considering maintenance and replacement costs.
Aircraft servoactuators are generally of two types: parallel and series, both of which are well known in the aircraft automatic control art. The parallel actuator is hard-mounted or bolted directly to the airframe and thus not severely affected by a high vibration environment as compared with conventional series actuator installations. A parallel actuator system is normally the autopilot actuator wherein its movement is reflected both at the control surface and the control stick. A series actuator, however, is not normally secured to the aircraft body structure but, rather is coupled in the control linkages or push-rod assemblies between the pilot's stick and the control surface; that is, it is floating or ungrounded and hence severely and adversely affected in the high vibration environment usually associated with a helicopter. The series servo motion does not move the pilot's stick and normally has a limited control authority over the control surface. Furthermore, as stated, the series actuator is usually installed in the control push-rods which, particularly in a retrofit situation, alters their natural resonances to local vibrations such as produced by the sustaining rotor.
A strapdown multifunction servoactuator which alleviates to a large extent the shortcomings of the above-described prior art is described in copending application Ser. No. 176,321, filed Aug. 8, 1980 entitled "Strapdown Multifunction Servoactuator Apparatus for Aircraft", by Carl D. Griffith and Kenneth L. Oliver and assigned to Sperry Corporation. The strapdown multifunction servoactuator of the above reference comprises one or two integral units adapted for installation in an aircraft, particularly a helicopter, and coupled between the pilot's control stick linkage and the aircraft control surface (or surface servo boost) linkage. The multifunction servoactuator performs the functions of series actuation, trim actuation, artificial feel, control position sensing, and control authority limits. The apparatus includes a series actuator installed in the vehicle and connected to the vehicle control linkages in the conventional parallel actuator manner; that is, it is secured directly or strapped down to the airframe and therefore greatly simplifies control system installation and reduces problems such as those associated with control rod vibration resonances normally encountered with conventional integral-with-linkage series actuator installations.
The series actuator of the above-described multifunction servoactuator, however, includes an external linkage assembly having a differential link which is an open framework coupled to a series servo link. The differential link has contact surfaces which contact contoured stop surfaces mounted on the apparatus housing. These surfaces are contoured such that when the manual command exceeds the authority limits, any motion of the series servo link produces a vertical motion of the output linkage point with negligible translational component. In order to accommodate the authority limit requirements of a large number of aircraft installations, various configurations of the contoured authority limit stop block must be available. It will be appreciated that this contoured stop block is rather complex and very expensive to manufacture. Furthermore, the open framework of the differential link may be too fragile and in some instances may not carry the imposed mechanical loads. Moreover, the open framework design is also difficult and expensive to manufacture. Accordingly, there is a need for less costly and less complex linkage assembly suitable for coupling a strapdown series servoactuator between a control surface and the pilot's control stick or feel trim actuator.